CMake: Major cleanup to support find_package(raylib)

Remove that link_libraries_to_executable() hack and defines a proper
raylib target that can be used with target_link_libraries.

The same target is also available for external (user) code by using
find_package(raylib).

This results in:

- Remove hardcoded build directories from examples and games CMakeLists.txt
- Allow rlgl_standalone and other special examples to be built easily
- Allow CMake projects to find_package(raylib instead of fiddling with pkg-config
- Makes code a little more maintainable
- Fixes #471, #606.
- Makes code less confusing by removing the double use of PLATFORM (#584).

Note that this is still not _The Right Way_(TM), because normally
raylib-config.cmake (or its includes) would be automatically generated.
I didn't manage to get that to work though, so I went the easier route
of just wrapping pkg_check_modules for consumption by find_package.

games/drturtle/CMakeLists.txt

@@ -1,18 +1,13 @@
-# Setup the project and settings
+cmake_minimum_required(VERSION 2.6)
 project(drturtle)
 
-include("../../cmake/utils.cmake")
-
-
-# Make sure raylib has been built
-# TODO `build` directory should maybe be something else...
-include_directories("../../build/release")
-
 # Executable & linking
-add_executable(drturtle 06_drturtle_final.c)
-link_libraries_to_executable(drturtle)
+add_executable(${PROJECT_NAME} 06_drturtle_final.c)
+if (NOT TARGET raylib)
+  find_package(raylib 2.0 REQUIRED)
+endif()
+target_link_libraries(${PROJECT_NAME} raylib)
 
 # Resources
 # Copy all of the resource files to the destination
 file(COPY "resources/"  DESTINATION "resources/")
-